Which of the following practitioners has limited ability to represent taxpayers before the irs?

The unenrolled preparers have limited ability to represent taxpayers before the IRS (The Internal Revenue Service).

This is because unenrolled preparers can only represent the tax payer for the tax return or claim of refund he personally prepared and signed.
This representation cannot be done before the revenue officers and can only be accepted by the revenue agents or employees of similar ranks in the IRA.